Apparatus for controlling and performing a microbiological or enzymatic plug flow process

ABSTRACT

To improve the yield and/or reduce the energy cost in carrying out a microbiological or enzymatic process in a reactor and to make the reaction conditions essentially independent of the size of the reactor, it is proposed on make use, as a reactor, of an endless circulation tube in which the reaction components are circulated essentially according to a plug flow and in this process are fed through one or more in-line mixers fitted inside the tube. This method and reactor are suitable in particular for the preparation by fermentation of polysaccharides, especially xanthan, in which water, a production medium containing one or more sugars and nutrient salts and an inoculating material of a suitable aerobic bacterium are introduced into the the reactor tube and exposed to fermentation with air being supplied. Preferably the concentration of a reaction component or a value derived therefrom is measured at at least one point and the reaction velocity is regulated on the basis of the the measurement within critical limits.

This is a divisional of copending application Ser. No. 06/796,919 filedon Nov. 12, 1985 now U.S. Pat. No. 4,935,398.

BACKGROUND OF THE INVENTION

The invention relates in the first instance to a method for the carryingout of a microbiological or enzymatic process in which reactioncomponents are fed into a reactor constructed as an endless circulationtube and a circulation current is brought about inside the said tube.

A method of this type is known from the published French PatentApplication 2,209,837.

It is usual to carry out microbiological or enzymatic processes in avessel provided with one or more stirring elements (see for example thepublished European Patent Application 0,111,253). It has become evidentthat when a reactor of this type is used, a considerable spread occursin the reaction time, i.e the time that a small element of liquidrequires in order to be conveyed around through the vessel starting fromthe stirrer and to be conveyed back to the stirrer. This spread has anunfavourable influence on the yield of the process since the liquidparticles with a relatively rapid circulation time will be subjected toobriefly to the treatment (inadequate conversion), whereas liquidparticles with a slow circulation time will be exposed too long to thetreatment. In certain processes the viscosity increases during thefermentation process and from a certain viscosity value upwards thestirrer generates a revolving cylinder while the remainder remainsessentially stationary. In the case of aerobic fermentation processes,to promote the multiplication of aerobic bacteria and their productformation, air is supplied to the reaction mixture. If a stirredcontainer is used, a relatively high oxygen concentration may occur atthe stirrer and a relatively low oxygen concentration at the vesselwall. These disadvantageous phenomena are intensified as the rheologicalproperties of the mixture change. In order, nevertheless, to achieve ascomplete a reaction as possible, high energy costs are often necessary,the energy being primarily used for the stirring. Another problem ofcarrying out a process in a stirred container is that enlargement(scaling up) of the equipment from the laboratory scale to theindustrial scale is accompanied by a considerable change in theconditions under which the process proceeds.

These drawbacks are not removed by the method according to the saidFrench Patent Application 2,209,837 since there are located inside thetube driven propellers which provide both for the circulation and forthe mixing by means of stirring. To this known method there also pertainthe disadvantages of high energy costs, relatively low yield anduncontrollable differences in oxygen concentration.

SUMMARY OF THE INVENTION

The object of the invention is to avoid the abovenamed drawbacks and toprovide a method referred to in the introduction, in which the reactionconditions on scaling up are essentially independent of the size of thereactor and the energy used is limited to a minimum.

According to the invention the method for this purpose is characterisedin that the reaction components are circulated in the completely filledtube by a plug flow and during this process are guided through one ormore in-line mixers fitted inside the tube.

The method according to the invention is, in particular, suitable forthe preparation by fermentation of polysaccharides, in particularxanth-an, production medium containing water, one or more sugars andnutrient salts and an inoculation material of a suitable aerobicbacterium being introduced into the endless reaction tube and the mediumin the said tube being exposed to fermentation with air being supplied.

If a constant circulation rate is used, very handsome results can beachieved which stand out far above the results of a stirred vessel. Itis, however, in general advisable to measure the concentration of areaction component or a value derived therefrom at at least one pointand on the basis of the said measurement to regulate the reaction ratewithin critical minimum and maximum limits in accordance with thekinetics of the process.

It is pointed out that from the published European Patent Application0,111,253 a method is known for the carrying out of a chemical reaction,in particular a biochemical reaction, which method is carried out in areactor vessel which is divided into two chambers by a wall; the vesselis not entirely filled and no plug flow is brought about. There is alsono mention of in-line mixers. The concentration of a reaction component,in particular of the reaction-retarding component, is indeed measureddirectly or indirectly and on the basis of the said measurement thesupply of one or more new components to the reactor is regulated in amanner such that a maximum is not exceeded.

By the reaction rate of an enzymatic or microbiological process is meantthe rate at which a certain degree of chemical conversion is reached.This may involve the rate of a certain oxygen absorption, carbonic acidproduction, heat generation, substrate consumption, product formationand the like. In general it is true that the reaction rate increases upto a certain concentration of a reaction component (C-minimum critical),then remains more or less constant up to a certain higher concentrationof the said component (C-maximum critical) and finally decreases atconcentrations of the said component which are still higher. It will beclear that it is beneficial that during the carrying out of a processthe concentration of a component is held between the critical minimumand maximum concentration values. By using the closed tube which iscompletely filled with reaction components and in which plug flow ismaintained inter alia by means of in-line mixers, the concentration ofthe component or of several components can be held between the criticalvalues by controlling the reaction rate by means of one or moreconcentration measurements or measurements of values derived therefrom.In particular, the rate of flow of the plug flow is suitable for beingcontrolled on the basis of the measurement or measurements of theconcentration of a component or a value derived therefrom. This rate offlow determines in fact the contact time between the different reactioncomponents, while transport parameters (gas-liquid; liquid-liquid;solid-liquid) are also determined by the rate of flow.

Besides opting for the regulation of the rate of flow of the plug flowit is possible to opt for the regulation of the rate of supply of asubstrate--this is a reaction component which is used as a nutrient. Inthe case of aerobic microbiological product formation one of thesubstrates consists of atmospheric oxygen. The consequence is that theconcentration of the substrate in the circulation tube directly behindthe mixer will be lower than the critical maximum value, whereasdirectly before the mixer, i.e. at the end of the circulation path, thisvalue must be higher than the critical minimum value. The reaction ratecan be controlled by supplying more or less substrate on the basis ofmeasurements of the substrate concentration with a carefully chosencirculation rate of the plug flow. To achieve an energy saving it is,however, preferable, with a carefully chosen substrate supply, tocontrol the reaction rate by regulating the rate of flow of the plugflow on the basis of measurements of the concentration of a reactioncomponent or of a value derived therefrom. Incidentally, the possibilityis not excluded that the reaction rate is controlled by the simultaneousregulation of the rate of plug flow and the supply of a reactioncomponent.

This process is influenced, inter alia, also by the number of substrateinjection points and the dimensioning of the static mixing elements andthe number thereof. For a given device, however, these are usually fixedand are therefore usually unsuitable for subjection to regulation.

It is essential for the effect of the invention that both theoptimization of the reactor and the control of the process conditionsare determined by the reaction kinetics.

The use of static mixers has the advantage that relatively little energyis used and the reaction volume can be relatively small, while theliquid is conveyed as a plug flow.

In addition to the preparation by fermentation of polysaccharides fromwater, glucose and nutrient salts with air as substrate and under theinfluence of aerobic bacteria, the invention can also be used for thepreparation of yeast from water, glucose and nutrient salts and a littleyeast. The possibilities also include the preparation of gluconic acidfrom glucose, the oxidation of ethene by micro-organisms and thepreparation of SCP (single cell protein) making use of paraffindispersed in water.

Instead of the concentration of a reaction component itself a valuederived therefrom may be measured, for which, depending on the process,inter alia the pH, the oxygen tension, the temperature and the like aresuitable.

The invention also relates to a reactor for the carrying out ofmicrobiological or enzymatic processes consisting of an endlesscirculation tube with means for the circulation of reaction componentsfed into the tube. A reactor of this type is known from the alreadymentioned French Patent Application 2,209,837. To be able to carry outthe method according to the invention an in-line mixer is fitted in atleast one section of the circulation tube, while the reactor is providedwith a circulation pump for bringing about a plug flow.

In order to be able to regulate the process between critical minimum andmaximum reaction rates, the reactor will have at least one measuringelement for the measurement of the concentration of one or more reactioncomponents or a value derived therefrom, while regulating means arepresent for regulating the pump speed depending on the measured value.Preferably the reactor is provided at at least one point with measuringelements for the measurement of the essentially maximum and theessentially minimum concentration of a reaction component or a valuederived therefrom.

A practical embodiment of the reactor embodies a vertical rising tube, avertical downtube and two horizontal connecting tubes, one or morestatic mixers being fitted at least in the rising tube, the circulationpump being fitted in the lowermost horizontal tube, a substrate supplyelement debouching into the bottom end of the rising tube, a substrateremoval element debouching into the top end of the downtube, a measuringelement for the measurement of the maximum substrate concentration or avalue derived therefrom being fitted in the top end of the rising tubeand a measuring element for the measurement of the minimum substrateconcentration or a value derived therefrom being fitted in the bottomend of the downtube.

In the preparation of polysaccharides, in particular xanthan, the liquidacquires structural properties, in particular pseudoplasticity, duringthe fermentation produced by the aerobic bacteria. In the case ofmixtures which become viscous a pump is always required for thecirculation. In the case of the preparation of microbial polysaccharidesin which the viscosity remains below a certain value, the circulationcan also be brought about by the injection of substrate.

The most important advantage of the invention is that for the sameenergy consumption the product yield is considerably higher than in thecase of a stirred vessel or a closed circulation tube according to thesaid French Patent Application 2,209,837 provided with propeller-shapedmixers. If a stirred vessel or the closed tube according to the Frenchapplication is used, because of the considerable rise in the viscosityit is necessary to stop at a point at which the product yield is stillrelatively low. This limitation exists to a much lesser extent in themethod according to the invention.

Embodiments of the reactors to be used in the method according to theinvention will now be explained in more detail with reference to thefigures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a side view, with a small section in cross-section, of afirst embodiment of the reactor according to the invention.

FIG. 2 shows a longitudinal section of a detail.

FIG. 3 shows a side view, with a small section in cross-section, of asecond embodiment of the reactor according to the invention.

FIGS. 4 and 5 show two graphs pertaining to Examples 2 and 3respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the two embodiments equivalent components are provided with the samereference numerals.

The reactor shown in FIGS. 1 and 2 consists of an endless circulationtube formed by a rising tube 1, a downtube 2, an uppermost horizontalconnecting piece 3 and a lowermost horizontal connecting piece 4. Thewhole is supported by a stand 5.

The rising tube 1 embodies a number of in-line mixers, preferablyconstructed as static mixers 6 which are able to mix the reactioncomponents without driven stirring elements by dividing the maincurrents, interchanging the positions of the partial currents andreuniting the partial currents again. Static mixers are inter aliadescribed in Dutch Patent Application 75.02953, 77.00090 and 80.04240.Sulzer SMV or SMX mixers are to be preferred. As is evident from FIG. 2,each static mixer is surrounded by a cooling/heating jacket 7 in which aheat transfer medium can be fed or removed through nozzles 8 and 9.Static mixer units may also be fitted in the downtube.

In the lowermost horizontal section 4 a circulation pump 11 isincorporated, for example constructed as a gear pump.

The reactor operates in general in a batch manner, although continuoussupply and removal of reaction components are not excluded. The reactoris first completely filled with the reaction component, i.e. in the caseof the preparation of polysaccharides by fermentation, a productionmedium which contains water, one or more sugars and nutrient salts andan inoculation material of a suitable aerobic bacterium. In the case ofxanthan this bacterium is Xanthomonas campestris ATCC 13951. After thefilling, the pump 11 is switched on to bring about a plug flow and airis fed in as substrate via the pipe 12. In the static mixers an intimatemixing of the reaction components takes place. The components arepartially consumed by the aerobic bacterial as a result of which thebacteria multiply and excrete a product. In the preparation byfermentation of polysaccharides atmospheric oxygen is made use of assubstrate and the said oxygen is consumed by the aerobic bacteria. Aftermixing excess gas will have to be separated off, which takes place atthe liquid-gas separator 16.

Between the static mixers the reactor may also embody intermediatepieces 17 through which certain components may be supplied.

Important advantages of the reactor constructed as a closed tube inwhich at least in one part of the tube in-line mixers are fitted and inwhich a circulation pump provides for the bringing about of a plug floware that the conditions under which the reaction takes place,independently of the size of the reactor, can be optimised and that theenergy consumption can be limited to a minimum. The scaling up of theprocess is facilitated by the fact that the course of the process in thereactor can be described, and consequently modelled, well. Microbialpolysaccharides have the property that they strongly influence therheology of the medium. In relation to a stirred vessel an energy savingis always achieved even if the circulation rate is chosen too low andthe oxygen is completely consumed before the product has reached thebottom of the downtube. In the preparation of polysaccharides goodresults can be achieved even with a constant circulation rate.

It is, however, preferable to make use of the reactor according to FIG.3 in which the concentration of a reaction component of a value derivedtherefrom is measured at one or more points and the reaction rate isregulated on the basis of the said measurement. In FIG. 3 measurementelectrodes 13 and 14 are disposed at the top end of the rising tube 1and at the bottom end of the downtube respectively.

These measurement electrodes are connected to a regulating unit 15 whichcontrols the pump 11 in a manner such that the reaction rate comes torest within critical minimum and maximum limits. In particular, themeasurement electrode 13 will be used to determine the maximumconcentration of substrate after mixing, whereas the purpose of themeasurement electrode 14 is to determine the minimum concentration ofsubstrate. To achieve an optimum reaction rate, the plug flow rate willbe adjusted by the pump in a manner such that the concentration of thesubstrate always comes to rest within a maximum and minimum criticalvalue. All this implies specifically that if the measurement electrode13 determines that the maximum concentration of substrate comes to restabove the critical maximum value, the pump speed will be reduced,whereas if the measurement electrode 14 measure$ that the minimumsubstrate concentration comes to rest below the critical minimum value,the pump speed will be increased.

The measurement electrodes measure the concentration of the substrate oranother reaction component itself or a value which is a direct functionof the said concentration, for which, depending on the process, interalia the pH, oxygen tension, the temperature and the pressure aresuitable.

In the embodiment shown in FIG. 3 the pump speed is regulated to allowthe reaction to proceed in an optimum manner. The possibility is notexcluded that the pump speed is constant and that the supply rate ofsubstrate and/or other reaction components is regulated on the basis ofmeasurements of concentrations or values derived therefrom. Injectioncan take place at more places and the number of injection points may bevaried on the basis of the said measurements. The possibilities alsoinclude regulation of the product removal rate.

The reactors described can be used for various microbiological and/orenzymatic processes. They are, in particular, suitable for theproduction of substances which strongly affect the rheology of themedium (for example, microbial polysaccharides). This is because theflow is well defined and can be kept constant the hydrodynamicconditions by varying the liquid flow rate.

EXAMPLE I

Xanthomonas campestris ATCC 13951 is cultivated at 30° C. on a tryptonglucose yeast extract agar for 48 hours. From a loosely disposed colonymaterial is inoculated into a flask containing glucose yeastextract-malt extract solution and suspended, after which cultivation iscarried out for 24 hours at 30° C. with shaking. 1 liter of thisinoculation material is added to 25 liters of fermentation mediumcontaining glucose as a carbon source in a concentration of 4% by weightand yeast extract as an organic nitrogen source in a concentration of0.5% by weight. Magnesium ions are added as MgSO₄ in a concentration of0.05% by weight. The pH is kept constant between 6.5 and 7.5 during thefermentation by adding KOH in a concentration of 2N. A basic buffer isused in the form of K₂ HPO₄ in a concentration of 0.2% by weight. Thismaterial was contained in a reactor tube as described above with avolume of 30 liters. The circulation time was 2 minutes so that thecirculation speed was 15 liter/minute. The temperature was 29° C. and 10liters of air were fed in per minute. The circulation of the materialwas continued for 72 hours. It emerged that 3% by weight of xanthan wasformed, 4 kW of energy being used per m³ of reactor volume. With thesame energy input (4 kW/m³) in a stirred vessel (on a 30 liter scale)the fermentation lasts 144 hours. The product concentration achieved isthen also 3% by weight. On a pilot plant scale this productconcentration is achieved in 144 hours with an energy input of 4-5 kw/m³using a stirred vessel. However, in 72 hours a much lower productconcentration, viz. 1.8-2.0% is obtained with this energy input.

EXAMPLE II

Xanthomonas campestris ATCC 13951 is cultivated at 30° C. on a tryptonglucose yeast extract agar for 48 hours. From a loosely disposed colonymaterial is inoculated into a flask containing glucose yeastextract-malt extract solution and suspended, after which cultivation iscarried out for 24 hours at 30° C. with shaking. 1 liter of thisinoculation material is added to 25 liters of fermentation mediumcontaining glucose as a carbon source in a concentration of 4% by weightand yeast extract as an organic nitrogen source in a concentration of0.5% by weight. Magnesium ions are added as MgSO₄ in a concentration of0.05% by weight. The pH is kept constant between 6.5 and 7.5 duringfermentation by adding KOH in a concentration of 2N. A basic buffer isused in the form of K₂ HPO₄ in a concentration of 0.2% by weight. Thefermentation is carried out for 65 hours at 30° C. in a reactor tube asdescribed above with a volume of 30 liters. During the fermentation theoxygen tension of the liquid measured by means of an oxygen electrode atpoint 14 is regulated to a value of approximately 15-25% of saturationwith air. For this purpose, by means of the signal from the saidelectrode, via suitable transducers, the speed of the pump motor and thequantity of air fed in via the connection at point 12, are regulated.

The course of this fermentation is shown in FIG. 4. This shows insuccession, as a function of the time, the viscosity of the fermentationmedium (expressed as Brookfield viscosity at 30 rpm measured with an LVTspindle), the oxygen tension in the solution (expressed as a percentageof saturation with air), the speed of the pump motor (expressed inrevolutions per minute), and the quantity of air supplied (expressed innormal liters of air per minute).

EXAMPLE III

Gluconobacter oxydans ATCC 621 H is cultivated for 24 hours on a slantagar tube containing a glucose yeast extract-chalk medium. From thistube all the bacteria material is inoculated into a flask of glucoseyeast extract-chalk solution and suspended and incubated for 12 hours at30° C. with shaking. 1 liter of this inoculation material is added to 25liters of fermentation medium containing a glucose (10% by weight) yeastextract (1% by weight) medium. During the fermentation the pH isregulated to 3.5 by adding NaOH in a concentration of 4N. Thefermentation is carried out in a reactor tube as described above with avolume of 30 liters for 15 hours at a temperature of 30° C. During thefermentation the oxygen tension of the liquid measured by means of anoxygen electrode at point 14 is regulated to a value of 15-25% ofsaturation with air. For this purpose, by means of the signal from thesaid electrode, via suitable transducers, the speed of the pump motorand the quantity of air supplied via the connection at point 12 areregulated.

The course of this fermentation is shown in FIG. 5. This shows insuccession, as a function of time, the concentration of gluconate in thefermentation medium (expressed in mmol of gluconic acid/liter), theoxygen tension in the solution (expressed as a percentage of saturationwith air), the speed of the pump motor (expressed in revolutions perminute) and the quantity of air supplied (expressed in normal liters ofair per minute).

We claim:
 1. Reactor for the carrying out of microbiological orenzymatic processes comprising a circulation tube, at least one staticin-line mixer inside said tube, a mechanical pump for the circulation ofreaction components fed into said tube, and a gas-liquid separator,wherein said circulation tube is a vertical tube loop having asubstantially vertical rising tube, a substantially vertical downtubeand two connector tubes connecting said riser tube and said downtube attheir respective upper and lower ends, the length of said connectortubes being small with respect to the length of said rising tube andsaid downtube, said at least one static in-line mixer having a designwhich ensures plug flow and being disposed inside said rising tube, andsaid gas-liquid separator having a position at the uppermost part ofsaid tube loop, said reactor further comprising first measuring meanspositioned at the upper end of said rising tube for measuring themaximum concentration of one or more reaction substrates or a valuederived therefrom, second measuring means positioned at the lower end ofsaid downtube for measuring the minimum concentration of one or morereaction substrates or a value derived therefrom, and regulating meansconnected to said pump, said first measuring means and said secondmeasuring means for regulating the speed of said pump in response to themeasured concentrations of said first and second measuring means.
 2. Thereactor according to claim 1, further comprising a substrate supplyelement extending in flow communication with into the lower end of saidrising tube.
 3. The reactor according to claim 1 further comprising asubstrate supply element extending in flow communication with into thelower end of said rising tube and a substrate removal tube extending inflow communication with into the upper end of said downtube, and whereinsaid mechanical pump is positioned in communication with the lowermostof said connecting tubes.